Surgical treatment of a degenerative spinal condition has for some time relied upon the attachment to consecutive spinal vertebrae of stabilizing, reinforcing plates. Such bone plates have been attached to the vertebrae by means of screws that are set into the bone. The surfaces of such plates bearing against the bone have been both flat and curved about a single axis. Attempts have been made to introduce specific configurations for such plates in order that they may fit more intimately against the bones they are engaging.
For background, reference may be made to an article in CLINICAL ORTHOPAEDICS AND RELATED RESEARCH Number 227, February 1988, page 135 entitled: "A Contoured Anterior Spinal Fixation Plate".
In terms of geometry, a spinal plate should be as wide as possible in cross-section in order to span the lateral side of the vertebrae to which it is attached. It should not protrude so as to interfere with blood vessels or nerve tissue. And preferably, in its long axis it should substantially follow the natural contour of the spine so that the vertebrae being reinforced may be held approximately in their normal orientation to each other.
It has been customary to produce spinal plates that are curved about a single longitudinal axis. Examples include U.S. Pat. Nos. 1,105,105; 3,695,259; 4,454,876; 4,493,317; and 4,683,878 and the plate described in the above Clinical Orthopaedics article. This curvature tends to create two parallel line-contacts between the plate and the vertebrae to which it is fastened so long as the cross-sectional radius of the plate is less than that of the vertebral body. Subject to irregularities in the vertebrae, such line contacts, when they occur at the outside edges of the spinal plate, create a maximum span between the opposed points of engagement with the vertebrae. This tends to improve and maximize the stability of the coupling between the plate and each vertebra.
While spinal plates in the past have been bent about a single axis of curvature, no successful attempts have been made, prior to this invention, to produce spinal plates that have two simultaneous forms or components of curvature.
Care must be taken in selecting the geometry of such plates to obtain an optimal compromise between features of shape and cost of manufacture. This is particularly true in the case of spinal fixation plates where a variety of different plate shapes are to be preferred, in accordance to the specific vertebrae to which they are to be attached.
It might generally be thought that the individual variety in human spinal geometry would optimally require that plates for fixation to specific individuals, should be custom shaped for that individual. This is not practical since no convenient means presently exists for the pre-surgical extraction of precise spinal dimensions; and spinal plates, at least those made of hard metal, such as plate steel, cannot be conveniently re-shaped during surgery.
While an ideal fit for each individual is not presently obtainable, it would be desirable to identify a preferred geometry for spinal plates that will allow such plates to be mass produced for large numbers of individuals, and still be reasonably close to the optimal geometry for each individual.
The challenge of defining a standard geometry for spinal plates is further complicated by the fact that spinal plates, particularly for the thoracic region, should preferably be manufactured with two separate curvatures embodied therein. As well, for the thoracic region, such plates should also be tapered in their width. In order to facilitate manufacture, it is desirable to reduce the geometry of such plates to a minimum of criteria that may readily be converted into manufacturing operations that can be carried-out by existing production machinery. Computer-controlled machine tools, operating on the basis of such criteria, may then be used to mass manufacture such plates.
It is accordingly an object of this invention to provide for a form of spinal plate that has a lower spine-opposing face that incorporates two distinct patterns or components of curvature.
It is further an object of this invention to define a template from which thoracic and lumbar spinal fixation plates may be cut which will fit, it is believed, a substantial proportion of human patients where such plates are required.
It is a still further object of this invention to provide a criteria for the distribution of screw holes on spinal plates that is optimal for the application for which such plates are intended.
The invention in its general form will first be described, and then its implementation in terms of specific embodiments will be detailed with reference to the drawings following hereafter. These embodiments are intended as examples to demonstrate the principle of the invention, and the manner of its implementation. The invention implicit in such embodiments, will then be further described, and defined, in each of the individual claims which conclude this Specification.